INTRODUCTORY ESSAY. 59 apartments it is preserved. As all the duplicates were made up into sets, ticketed, and distributed at home and abroad, this herbarium has taken the place of a standard work of reference, and it is impossible to over-estimate its value, or the importance of the constant access which we have enjoyed to its contents. The numbers attached to each plant have been so cited by all monographists, that a reference to these, in the great jnajority of instances, suffices for the identifica- tion of the species -,, and we have therefore constantly quoted the catalogue numbers, carefully examining every specimen before doing so, iii order to avoid as much as possible the risk of error. The distribution appears on the whole to have been made with much care, though the limited time allotted to its execution prevented that critical comparison without which species of difficult genera cauuot be discriminated. Hence we occasionally find two or more species under the same num- ber and letter, and far more frequently the same species under two or, more numbers. It is not easy to say how many spe- cies are contained in the WalUcliiau collection; but the 9000 mnul>crs may, we think, be diminished by at least one-fourth, as Dr. Wallidi, being obliged to distribute without describ- ing, very judiciously avoided uniting apparently distinct forms. For the present therefore we estimate this great collection at between 6500 and 7000 species. The named specimens of this Herbarium having bccu, as we have said, extensively dis- tributed, it has been customary with bo taunts to retain the names given by Dr. Wallich. We have been careftd to do the same ourselves for all otherwise unpublished genera and fcipecieaj but where published names, accompanied with de- scriptions, have come in contact with them, we have consi- dered it to be our duty to follow the generally recognized rule of priority, and to retain the published one; cxcep^ of coarse, iu oases where the authors of these names had habitually availed themselves of the Walliehian collections, and where we feel justified in assuming that they would wish to have adopted the Walliehian name had they recognized the plant.